The present invention is generally in the field of optical monitoring techniques, and relates to the control of semiconductor processing by measuring parameters of thin films, e.g. within processing equipment (the so-called xe2x80x9cintegrated metrologyxe2x80x9d).
Optical methods for on-line or integrated measurement of the parameters of dielectric films (e.g., film thickness) are known in the art. Most of these techniques are based on reflectometry in broaden spectral range, e.g. from DUV to NIR spectral range.
The NanoSpec 9000 spectrophotometric device commercially available from Nanometrics Inc., USA (that is installed on the CVD production cluster tool Producer commercially available from Applied Materials, USA) utilizes a configuration that allows measurements of a wide range of dielectric films just after deposition within the CVD cluster tool before a processed wafer goes to an out-put cassette. The device includes a measuring tool installed in a load/unload block, outside a vacuum part of the cluster tool. This configuration suffers from that it applies measurements to a wafer a certain time after the wafer moves out of a reaction deposition chamber. During this time period, which is needed for transferring the wafer within the vacuum part of the cluster tool and out of this vacuum part towards the cassette block, other wafers of the lot pass through the deposition chamber. This time delay impedes appropriate control of the manufacturing process. Actually, in case of malfunction or drift effect in the deposition process, measurements by a tool located out of the vacuum environment of the CVD tools arrangement will recognize this effect with a certain delay, and wafers of the lot processed after the first measured wafer will be scrapped.
Another technique is used in the NovaScan 840 integrated measuring tool, commercially available from Nova Measuring Instruments Ltd. According to this technique, a station (integrated metrology tool) installed on the CVD cluster tool as a separate vacuum chamber, or a non-operated chamber of the cluster tool is used for measurements. This is implemented by locating a measuring optical system outside the vacuum environment and separated therefrom by an optical window made in the respective chamber. The optical system utilizes a spectrophotometric measuring unit that measures the thickness of a deposited film through the optical window without affecting the deposition process. The measurements are performed on predetermined measuring sites in the wafer with a relatively small illuminating/measuring spot. The typical spot size used in the system is about 15-20 xcexcm in diameter. This configuration allows recognizing process deviations just after the first processed wafer is transferred from the processing area or chamber to the measuring area (chamber) and is measured by the integrated metrology tool. Such a fast response allows for xe2x80x9con-linexe2x80x9d process controlling and correcting the processing parameters for the next wafer to be processed or to stop the processing at all if needed prior to processing the next wafer.
Since the above system utilizes a relatively small measuring spot and performs measurements on the predetermined sites, it requires precise positioning of the optical system relative to the wafer under measurements, as well as pattern recognition and auto-focusing techniques. A precise positioning means is used to locate the small spot on the predetermined test site using a predefined optical model (properties of all or at least some of the underneath layers of the wafer). Knowledge of the optical model allows accurate and unambiguous interpretation of the measured reflectance spectrum. However, this system suffers from the need for a time consuming alignment (e.g. pattern recognition, auto-focusing, and precise positioning) and additional operations or steps within the cluster tool associated with the wafer handling by an internal cluster robot that might affect the cluster tool operation sequence and its throughput, especially in case of deposition of very thin films with short deposition time and respectively with high throughput of the cluster tool.
Still another approach for integrated measurements of the films"" thickness, particularly applicable to vacuum processing tools, consists of using a relatively large measuring spot (e.g., PCT publication No. WO00/12958 in the name of TEVET, or U.S. Pat. No. 5,900,633 in the name of On-Line Technologies Inc.). Such a technique does not require any pattern recognition, auto-focusing, precise positioning of a wafer, and/or movement of the optical system. Thus, the entire measuring cycle may be sufficiently reduced in order not to affect the throughput of the processing tool. Moreover, this technique provides measurements carried out during the wafer transfer from one location to the other within the processing (e.g. cluster) tool.
Measurements with a relatively large spot are implemented by averaging reflected light from a relatively large wafer""s area (e.g. of a diameter d=20-30 mm), i.e. slightly larger than the typical diagonal size of a die. Interpretation of the measured data is significantly different from that utilized in the above-indicated small light spot based technique (e.g. 15-20 xcexcm). Averaging of reflections from different elements of the wafer pattern within a large light spot covering different optical stacks with unknown weighting makes spectrum analysis and data interpretation very difficult, especially in those cases where there is a number of underlying layers in the wafer. Such a technique in case of multi-stack structures suffers from low confidence and low accuracy. In some cases, the contribution of the measured top layer within the relevant stack is so small that the measured reflectance spectrum is practically insensitive to this layer and cannot be measured with desired accuracy.
There is accordingly a need in the art to facilitate optical measurements of parameters of a patterned structure, such as a semiconductor wafer, by providing a novel optical system enabling measurements with measured areas of different sizes.
The main idea of the present invention consists of combining the advantages of both xe2x80x9clarge-spotxe2x80x9d and xe2x80x9csmall-spotxe2x80x9d approaches. By integrating a measurement system of the present invention with a processing tool, the accurate thickness measurements of a wafer""s layer(s) can be provided with minimal effect on the throughput of a processing tool.
There is thus provided according to one broad aspect of the present invention, an optical system for use in a measurement system for measuring in patterned structures, the system comprising:
(i) an illuminator unit producing an illuminating beam of light to be directed to the structure to produce a light beam returned from the structure;
(ii) a detector unit comprising an imaging detector and a spectrophotometer detector; and
(iii) a light directing assembly for directing the illuminating beam to the structure and directing the returned beam to the detector unit, the light directing assembly defining a first optical path for the light beams propagation, optical elements accommodated in the first optical path affecting the light beam to provide a relatively small measured area, and a second optical path outside said first optical path, such that the light beams propagation through the second optical path provides a relatively large measured area, as compared to that of the first optical path.
The term xe2x80x9cmeasured areaxe2x80x9d used herein signifies a region on the structure""s plane as viewed by the detector. This measured area is defined by the properties of the light directing optics and the sensitive area of the detector. The terms xe2x80x9csmall spot operational/measurement modexe2x80x9d and xe2x80x9clarge spot operational/measurement modexe2x80x9d signify system operations with, respectively, relatively small and large measuring areas.
In one embodiment of the invention, the optical elements in the first optical path include an objective lens that focuses the illuminating beam onto the structure, while the second optical path is defined by an optical arrangement that is accommodated upstream of the objective lens with respect to the direction of the illuminating beam propagation towards the structure""s plane, and is shiftable between its operative position being in the optical path of the light beam propagating towards the objective lens and inoperative position being outside the path of the light beam propagating towards the objective lens. Hence, when the optical arrangement is in its operative position, it directs the illuminating beam (and returned beam) to propagate along the second optical path aside the objective lens, thereby providing a relatively large measured size, and when the optical arrangement is in its inoperative position, the illuminating beam (and returned beam) propagates through the objective lens, thereby resulting in a smaller measured area.
Preferably, the optical arrangement comprises first and second spaced-apart mirrors facing each other by their reflective surfaces. The first mirror is mounted stationary aside the objective lens, and the second mirror is movable between its inoperative position being outside the optical path passing through the objective lens and its operative position being inside said optical path. The optical arrangement may additionally comprise a beam-expanding unit accommodated in the path of a light beam reflected from the first mirror.
In another embodiment of the invention, the optical system comprises at least two optical sub-systems, either utilizing a common illuminator and/or detector or not, wherein one sub-system is designed to provide a smaller measured area, and at least one other sub-system is designed to provide a larger measured area.
There is thus provided according to another broad aspect of the present invention, an optical system for use in a measurement system for measuring in patterned structures, the system comprising:
(i) an illuminator unit producing an illuminating beam of light to be directed to the structure to produce a light beam returned from the structure;
(ii) a detector unit comprising an imaging detector and a spectrophotometer detector; and
(iii) a light directing assembly for directing the illuminating beam to the structure and directing the returned beam to the detector unit, the light directing assembly defining a first optical path for the light beam propagation, optical elements accommodated in the main optical path affecting the light beam to provide a relatively small measured area, and a second optical path outside said first optical path, such that the light beam propagation through the second optical path provides a relatively large measured area, as compared to that of the first optical path.
According to yet another broad aspect of the present invention, there is provided a processing tools arrangement comprising a processing tool defining a processing region, and an integrated measurement system having the above-described optical system associated with a region within the processing tools arrangement outside said processing region.
The present invention also provides according to its yet another aspect, a method for controlling a process applied to a patterned structure progressing on a production line, the method comprising selectively applying optical measurements to at least one predetermined site on the structure with measured area of different sizes.